Legal Question in Real Estate Law in California

The California Probate Code allows there to be multiple attorneys-in-fact (i.e., holders of POAs) with respect to a single person, but the holders of the powers of attorney must act unanimously. If the POAs indeed both are general or both cover real-property sales, neither of you can sell the real property without the consent of the other. See Probate Code section 4202, especially subsection (b), "Authority granted to two or more attorneys-in-fact is exercisable only by their unanimous action."

Although Mr. Whipple is correct in the event there are two valid POAs, based on your question there appear to be zero valid POAs. A mentally incompetent person cannot give a valid POA, and not only that, if a person gives a POA while mentally competent and then becomes mentally incompetent the POA expires, unless the POA is "durable" and a medial professional declares the person incompetent and subject to the durable POA. So unless your POA is a durable POA that by its terms becomes or remains valid in the event of your mother's incompetence, neither POA is valid anymore. It sounds like it is time to get your mother a conservator.